The economic viability of marginal petroleum wells depends on the well's product flow and pressure capacity and the rate at which undesirable liquids (i.e., brine) infiltrate the well casing. A number of patents have been issued over the past 50 years addressing oil and gas well swabbing devices that offer the potential for unattended self actuation in an operating well environment. None of these inventions have proven to be operationally acceptable.
One known device is an airlift system, which features a cylindrical pumping device through which the fuel product flows. Flow in the annular passage between the cylindrical device and the well casing or tube walls is eliminated by closing off this area with flexible friction cups (rubber like material) or other mechanical means. A valve controls the flow of liquid and gas through the cylindrical pump. When the valve is closed the well is effectively shut-in. In other words, when the valve is closed the cylindrical pumping device seals the well closed. The resulting pressure build-up below the pump lifts the pump and the liquid above it to the surface. The flow capacity and shut-in pressure capability of the well must be sufficient to accomplish the lift. Upon reaching the well head, the control valve in the pump, is mechanically forced open to release the shut in pressure. The fluid below the pump then flows through the pump and out of the well.
Two basic approaches have been used in prior devices to close the flow control valve after the cylindrical pump has reached the desired location in the well. In some applications the flow control valve is forced closed by the impact of the pump striking a fixed stand located in the well. Situations develop operationally, however, where the fluid above the stand rises to a level that is too high for the subsequent shut-in pressure of the well, and the lift can not be accomplished. When this occurs, the well continues to be shut in until the cylindrical pumping device is mechanically retrieved from the bottom of the well.
In other applications, such as in the device disclosed in U.S. Pat. No. 4,986,727 of Blanton, the valve is closed when the well pressure is sufficient to overcome the resistance of a pressurized bellows in the device. This well pressure (set point pressure) is composed of both the flow pressure (also referred to as back pressure or casing pressure) and the hydrostatic pressure resulting from the column of liquid above the control valve.
Because the pumping device does not sense liquid level but is pressure activated, the control valve closes whenever the pressure at the valve reaches the set point pressure. Also, regardless of the pressure level in the well, the pumping device will descend into the well whenever the pressure differential across it (top to bottom) decrease to less than about 10 PSI. When the control valve is mechanically forced open, at the well head, the pressure differential across the swabbing device approaches zero.
Field experience indicates that when the control valve is forced open the unattended pumping device routinely drops down the well while the well was still flowing at very high pressure following shut-in. This has resulted in erratic operation, partial fluid lifts, valve cycling, and dry device lifts that have caused damage to the pumping device as well as to the supporting equipment at the well head. To prevent this, it was found necessary to hold the pumping device at the well head until the casing pressure dissipated to a normal operating level. This has required the design of automated latching devices to restrain the pumping device at the surface, using either maintenance personnel or timing devices to activate a release when the casing pressure is reduced to an acceptable working level. These solutions have added to the cost and complexity of the installations.